Imaging agents for use in magnetic resonance blood flow/perfusion imaging

ABSTRACT

In one aspect, a method of imaging blood flow in a region of interest of a subject using magnetic resonance imaging (MRI) is provided. The method includes introducing an imaging agent into the subject, the imaging agent including a compound having at least one hyperpolarized nucleus having a T 1  greater than 15 seconds and a water-octanol partition coefficient between −1 and 1, providing at least one excitation signal to the region of interest, the at least one excitation signal configured to invoke a nuclear magnetic resonance (NMR) effect at least in the introduced imaging agent, and detecting an NMR signal emitted by the region of interest in response to the at least one excitation signal. In another aspect, the imaging agent includes a carbon-13 enriched alcohol.

RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Ser. No. 60/814,945, entitled “CARBON-13 LABELEDALCOHOLS FOR USE IN BLOOD FLOW IMAGING WITH HYPERPOLARIZATION-ENHANCEDMAGNETIC RESONANCE TECHNIQUES,” filed on Jun. 19, 2006, which is hereinincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to imaging agents for use in blood flowimaging procedures via magnetic resonance imaging (MRI).

BACKGROUND

Tracking blood flow into tissue (“perfusion”) is an important tool formedical diagnosis and research. Tissues require blood in order tosurvive and function; a decrease in perfusion (“ischemia”) is theprinciple mechanism in many of the major causes of mortality andmorbidity, including myocardial ischemia, pulmonary embolism, andstroke. Perfusion also plays a critical role in cancer development. Atumor's capacity to augment blood flow by means of angiogenesis is a keylimiting factor in cancer progression. Accordingly, diagnostic imagingtechniques that can accurately and sensitively measure blood flow areneeded for clinical and research studies.

Conventional methods for blood flow imaging have been proposed for avariety of imaging modalities including Positron Emission Tomography(PET), x-ray computed tomography (CT) and magnetic resonance imaging(MRI). PET methods include tracking radioisotope-labeled agents. Theseagents, added to the blood, emit positrons that are detectable by thePET scanning apparatus. In CT, iodated contrast agents are traditionallyprovided to the blood stream. The iodated contrast agents enhance thex-ray attenuation characteristics of the blood, increasing the contrastbetween the blood and surrounding tissue in the resulting CT image.

SUMMARY OF THE INVENTION

Some embodiments include a method of imaging blood flow in a region ofinterest of a subject using magnetic resonance imaging (MRI), the methodcomprising introducing an imaging agent into the subject, the imagingagent including a compound having at least one hyperpolarized nucleushaving a T₁ greater than 15 seconds and a logarithm of the water-octanolpartition coefficient between −1 and 1, providing at least oneexcitation signal to the region of interest, the at least one excitationsignal configured to invoke a nuclear magnetic resonance (NMR) effect atleast in the introduced imaging agent, and detecting an NMR signalemitted by the region of interest in response to the at least oneexcitation signal.

Some embodiments include a method of imaging blood flow in a region ofinterest of a subject using magnetic resonance imaging (MRI), the methodcomprising introducing an imaging agent into the subject, the imagingagent including a carbon-13 enriched alcohol, providing at least oneexcitation signal to the region of interest, the at least one excitationsignal configured to invoke a nuclear magnetic resonance (NMR) effect atleast in the introduced imaging agent, and detecting an NMR signalemitted by the region of interest in response to the at least oneexcitation signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D illustrate various configurations of a tert-butyl alcohol(t-butanol) imaging agent, in accordance with some embodiments of thepresent invention;

FIGS. 2A and 2B are graphs illustrating the integrated signal for asingle saturation recovery experiment obtained using deuteratedt-butanol with carbon-13 enhancement of the central carbon, inaccordance with some embodiments of the present invention;

FIGS. 3A-3D illustrate various configurations of a 2-methyl-2-butanolimaging agent, in accordance with some embodiments of the presentinvention;

FIG. 4 illustrates a scheme for preparing hyperpolarized2-methyl-2-butanol by means of para-hydrogen induced polarization, inaccordance with some embodiments of the present invention; and

FIG. 5 is a flow chart illustrating a method of using an imaging agentto facilitate imaging blood flow/perfusion, in accordance with someembodiments of the present invention.

DETAILED DESCRIPTION

Despite the need for obtaining quality images of blood perfusion invarious portions of the body, conventional methods of blood flow imagingoften have drawbacks. Radioisotope-labeled tracking agents used in PETscanning pose significant safety hazards and typically offer onlylimited resolution and sensitivity. Iodinated contrast agents employedin CT scanning are typically incapable of penetrating the blood-brainbarrier and therefore may not be suitable for blood flow imaging ofbrain tissue or suspected tumors. Moreover, although the rate ofiodinated contrast arrival into other tissues can depend on perfusion,the rate that the compounds leave the blood stream is stronglydetermined by the natural permeability of capillaries. The combinationof both perfusion and permeability makes it difficult to obtain reliableresults from CT blood flow imaging studies. Background sources of signalmay complicate the analysis as well. In particular, high backgroundsignals may limit the true signal-to-noise ratio (SNR) of the imagingprocess because motion and other artifacts that are typicallyproportional to the background signal intensity can dominate the noiseof the measurement.

Magnetic resonance imaging (MRI) is a technique frequently used forblood flow/perfusion imaging. MRI is based on detecting nuclear magneticresonance (NMR) signals, which are electromagnetic waves emitted byatomic nuclei in response to state changes resulting from appliedelectromagnetic fields. In particular, magnetic resonance (MR)techniques involve detecting NMR signals produced upon the re-alignmentor relaxation of the nuclear spin of atoms in the body (e.g., in blood,tissue, etc.). MRI operates by manipulating the spin characteristics ofmolecules and includes aligning the spins of nuclei with an axialmagnetic field B₀, and then perturbing the magnetic field in a targetedregion with one or more radio frequency (RF) magnetic fields B₁.

The NMR phenomenon results from exciting nuclei by generating RF signalsB₁ at the Larmor frequency and applying them to a region of interest.The Larmor frequency is related to the rate at which nuclear spinsprecess about an axis at a given strength of the axial magnetic fieldB₀. When placed in magnetic field B₀, the spin of atomic nuclei align inone of two configurations: 1) a low energy configuration in which spinsalign in the opposite direction of the magnetic field (i.e.,anti-align); and 2) a high energy configuration in which spins align inthe same direction as the magnetic field, with more atoms aligning withthe magnetic field than anti-aligning at equilibrium (i.e., the state ofthe nuclear spins in the presence of applied magnetic field B₀ alone).

An applied RF magnetic field B₁ causes the nuclear spins to changeorientation, and causes some nuclei to achieve a higher energy state.When the RF signal B₁ subsides, the nuclear spins realign with the axialmagnetic field B₀. Those nuclei that achieved the higher energy stateupon excitation, return to the lower energy state by releasingelectromagnetic energy. The released electromagnetic energy may bedetected as NMR signals and used to form one or more imagesrepresentative of the tissue type in the region of interest. Thus, thepopulation difference between the number of aligned and anti-alignednuclei is related to the strength of the NMR signal released uponrelaxation. The NMR signals may be detected using one or more RF coilssensitive to electromagnetic changes caused by the NMR signals.Similarly; the NMR phenomenon may be invoked in part, using RF coilsthat generate an excitation RF pulse sequence. Numerous pulse sequencessuitable for obtaining NMR signals are known, and will not be discussedin detail herein.

Conventional MR techniques for blood flow imaging may be unsuitable fora number of reasons. Many conventional MRI contrast agents consist oflarge molecules, or chelates that are not capable of flowing into mosttissues. As a result, these contrast agents may be of limited or no usein certain parts of the body. Attempts at using smaller size moleculesas imaging agents that may effectively flow into a wider range oftissues have been frustrated by either toxicity of the agents or theinsufficient signal intensity resulting from the agents, making theiruse generally impractical. Another drawback of conventional MRI contrastagents include relatively short spin-lattice relaxation times (T₁),which limit the time the agent has to penetrate a region of interest andaccumulate so as to generate sufficient signal strength to measuretissue perfusion.

Hyperpolarization techniques offer a means to create useful signals evenwith smaller agents, but existing efforts have not yieldedhyperpolarized substances that are practical to employ as generalperfusion agents. Hyperpolarization increases SNR by increasing thepopulation difference between nuclei that attain the higher energy stateand lower energy state in equilibrium. As discussed above, thepopulation difference is proportional to the NMR signal strength andincreasing the population difference has the effect of increasingemitted NMR signal strength.

An early technique using hyperpolarization to facilitate blood flowimaging in MRI relied on the laser excitation of certain noble gases(e.g., either xenon or the 3He isotope of helium). 3He is not soluble inblood and accordingly may not be suitable for tracking perfusion. Xenonis soluble in blood, but is not satisfactory for several reasons. First,the gas interacts with lipids in tissues, thereby causing large shiftsin frequency and shortening T₁, thus reducing the time it takes nucleito reequilibrate after hyperpolarization. Relaxation times are typicallyless than 10 seconds in blood, and as little as 2 seconds in tissue.Accordingly, the T₁ may be too short to facilitate imaging bloodperfusion.

Because the maximum concentration of xenon in blood is quite small(e.g., to avoid anesthetic effects such as unconsciousness), it takes arelatively long time for an adequate concentration of the gas toaccumulate in tissue (e.g., more than one minute). Therefore,hyperpolarized xenon may not be preserved long enough for a sufficientamount of the agent to reach the area of interest and accumulate there.In addition, the concentration of xenon necessary for the MRI procedureis relatively high (e.g., typically more than fifty percent of theconcentration sufficient to cause patients to lose consciousness). Yetanother concern arises from the fact that xenon's interaction withlipids, as in red blood cells, may slow the rate of release into thetissue of interest, essentially making xenon a diffusion limited tracerat high flow.

Applicant has appreciated that the problems encountered with xenonevidence a need for agents that have a relatively long T₁, generally lowtoxicity (e.g., such that concentrations in the 1 to 10 mM range in thebrain and other tissues are acceptable), and an ability to pass readilyinto lipid membranes in the walls of blood vessels and cells withoutexcessive interaction with the lipids in the membranes themselves. Forexample, Applicant has appreciated that imaging agents that are bothsoluble in water and lipids may facilitate generally free diffusion ofthe agents across lipid membranes. In addition, the background level ofsuch tracers in the body should be minimal to avoid reducing the SNR ofthe signal.

New opportunities for the creation of agents having one or more of theabove beneficial characteristics have arisen from the development ofseveral “ex vivo” methods of hyperpolarizing carbon-13 enrichedcompounds and then injecting them into the sample of interest. One suchtechnique, referred to as dynamic nuclear polarization (“DNP”), relieson the freezing of the imaging agent in a glass state, irradiating thematerial with microwaves, and then rapidly warming the agent. DNP hasbeen used to increase the signal from carbon-13 enriched liquids byseveral orders of magnitude. A more detailed discussion of various DNPtechniques are described in U.S. Pat. No. 6,453,188, which is hereinincorporated by reference in its entirety. A second technique, referredto as para-hydrogen induced polarization (“PHIP”), involves the reactionof para-hydrogen (H₂ in the singlet spin state) with an imaging agentprecursor containing carbon-13 to produce a hydrogenated imaging agent.A more detailed discussion of PHIP techniques are discussed in U.S. Pat.No. 6,574,495, which is herein incorporated by reference in itsentirety.

Applicant has appreciated that the use of a certain class of imagingagents may remedy one or more of the above described problems associatedwith conventional imaging agents. In particular, Applicant hasidentified a class of imaging agents for use in MRI imaging as a bloodflow tracer that can be hyperpolarized and injected or inhaled. Certainmolecules in the class of imaging agents are generally soluble in bothwater and lipids, facilitating perfusion imaging in a wide range oftissues including, but not limited to, brain tissues which have beendifficult to image with respect to blood flow (due in part to the bloodbrain barrier). In addition, some compounds according to the presentinvention have relatively long T₁ times suitable for blood flow imaging.

In some embodiments according to the present invention, carbon-13enriched alcohols are used as an imaging agent to obtain one or more MRIimages of blood perfusion. Carbon-13 enriched alcohols represent aneffective class of compounds to hyperpolarize via DNP or PHIP and inturn use for MRI perfusion studies. Alcohols are highly soluble in bothwater and lipids. In particular, they can cross the blood-brain barrieras well as cellular membranes, fostering high concentrations in tissueand ensuring that little or none of the magnetic resonance signal willbe attributable to background concentrations. The cellular permeabilityalso increases the mean transit time in the tissue, thereby decreasingthe required temporal resolution for imaging and allowing for slowerinjection of the tracking agent.

Carbon-13 enriched alcohols may offer several practical advantages aswell. Alcohols are relatively nontoxic, and their bio-distribution hasbeen extensively studied. Accordingly, the effects of alcohol in thebody are well known and relatively safe at contemplated levels. Innon-enriched tissues, moreover, the background signal of carbon 13 isalmost zero, thus reducing noise and increasing SNR. Carbon-13enrichment of alcohols is a known and relatively standard procedure, andmany basic alcohols are available in enriched form as catalog items,making such agents readily available to the medical community withoutrequiring customized production.

Applicant has identified a number of compounds that can be used asimaging agents that facilitate obtaining images of blood flow/perfusion.FIGS. 1A-1D illustrate various configurations of a tert-butyl alcohol(t-butanol) molecule that may be used as a contrast agent to image bloodflow, in accordance with some embodiments of the present invention. FIG.1A illustrates the t-butanol molecule enriched with carbon-13 at thecentral tertiary carbon. This configuration may result in a particularlylong T₁ due, in part, to the four bonds with non-magnetic nucleishielding the central carbon-13 atom from the magnetic moments of theoutlying proton magnetic moments. Alternatively, the three carbons inthe outlying methyl groups may be enriched with carbon 13 as illustratedin FIG. 1B. Other configurations of enriching t-butanol with carbon-13may be used as well (e.g., combinations of enriching the outlying andcentral carbons), as the aspects of the invention are not limited inthis respect. Any of the above described molecules may be a suitablecandidate for DNP to hyperpolarize the molecules to increase NMR signalstrength.

The T₁ of the t-butanol molecule can be further increased by replacingthe outlying hydrogen atoms with deuterium as illustrated in FIG. 1C. Ithas been determined that a deuterated sample of t-butanol with carbon-13enrichment of the tertiary carbon has a T₁ in excess of 20 seconds at400 Mhz when measured in water with oxygen present. Although the T₁ ofdeuterated t-butanol with carbon-13 enrichment of the methyl groups(e.g., as illustrated in FIG. 1D) is shorter, this species may beadvantageous because of the enriched signal strength resulting from thethree carbons on each molecule. The median value of T₁ at 400 Mhz, 9.6T,for deuterated t-butanol with carbon-13 enhancement of the centralcarbon was 64±20 sec, and T₁ for deuterated t-butanol with carbon-13enhancement of the methyl carbons was 26±5 sec. FIGS. 2A and 2B aregraphs illustrating the integrated signal for a single saturatedrecovery experiment obtained using deuterated t-butanol with thecarbon-13 enhancement of the central carbon, in which T₁=46±4 andT₂=0.6±0.02. The relatively long T₂ may yield further SNR enhancements.

As discussed above, carbon-13 enriched alcohols may be hyperpolarized toincrease NMR signal strength. For example, one procedure for DNP oft-butanol includes mixing the liquid alcohol with a free radical such as2,2,6,6-tetramethyl-piperidine-1-oxyl, then rapidly freezing the mixtureby dripping through cold gas. The agent is hyperpolarized by subjectingit to microwave irradiation at low temperature, then prepared forinjection by rapid warming, for example, using the water methoddisclosed in U.S. Patent Publication No. 2002/0058869, which is hereinincorporated by reference in its entirety. Although the free radical maybe filtered out, its toxicity at concentrations contemplated forperfusion imaging is not high. Further detail describing DNP ofdeuterated butanol are described in Nuclear Instruments and Methods inPhysics Research A 400:133-136 (1997), which is herein incorporated byreference in its entirety. DNP of t-butanol or deuterated butanol may beperformed according to other methods, as the aspects of the inventionare not limited in this respect.

FIGS. 3A-3D illustrate various configurations of a 2-methyl-2-butanolmolecule Suitable as an imaging agent, in accordance with someembodiments of the present invention. FIG. 3A illustrates2-methyl-2-butanol enriched with carbon 13 at the central tertiarycarbon, however, the two adjacent methyl groups may be enriched withcarbon 13, either alone or in combination with the central tertiarycarbon. For example, FIG. 3B illustrate the two methyl groups enrichedwith carbon-13 in place of the central carbon. As with t-butanol, T₁ ofthe molecule can be further increased by replacing the outlying hydrogenatoms with deuterium as illustrated in FIGS. 3C and 3D. Any of the abovedescribed molecules may be suitable candidates for hyperpolarizationusing PHIP, which can be readily synthesized by the hydrogenation ofdimethyl vinyl carbinol in the presence of a rhodium catalyst, asdiscussed in J. Organomet. Chem. 570:63-69 (1998), which is hereinincorporated by reference in its entirety.

FIG. 4 illustrates a scheme for preparing hyperpolarized2-methyl-2-butanol by means of para-hydrogen induced polarization. Inparticular, 2-methyl-3-buten-2-ol (dimethyl vinyl carbinol) ishydrogenated with para-hydrogen to form the hyperpolarized2-methyl-2-butanol. While dimethyl vinyl carbinol provides a suitableprecursor, other precursors may be used, as the aspects of the inventionare not limited in this respect. Following the reaction of the carbon 13enriched substrate with para-hydrogen gas in the presence of the rhodiumcatalyst, the agent may be filtered if necessary to remove the catalystand any remaining, non-hydrogenated substrate.

Hyperpolarized t-butanol and 2-methyl-2-butanol may offer severalparticular advantages as an imaging agent. As discussed above, thehyperpolarized molecules have relatively long relaxation times that maybe long enough for transfer, injection, distribution and imaging. Inaddition, both molecules may be freely diffusible across lipid membranesdue, in part, to the property that the molecules are soluble in bothwater and lipids. A common measure of the solubility of a compound ormolecule is the water-octanol partition coefficient. The water-octanolcoefficient is a measure of the differential solubility of the compoundbetween the two solvents. While other solvents may be used, water andoctanol are useful to measure how hydrophilic (e.g., the extent of thesolubility of the compound in water) and how hydrophobic (e.g., theextent of the solubility of the compound in octanol) the compound is.The solubility in octanol is an indicator of how lipid soluble thecompound is, and may therefore be a useful measure of how freely acompound diffuses across, for example, the blood brain barrier, or otherlipid membranes. The water-octanol partition coefficient may be computedas:

$\begin{matrix}{{\log\left( \frac{{Solute}_{{oc}\mspace{11mu} \tan \mspace{11mu} {ol}}}{{Solute}_{water}} \right)}.} & (1)\end{matrix}$

Determining the water-octanol partition coefficient for a molecule iswell known and will not be discussed in detail herein. Molecules thatare soluble in both water and lipids such that they are relativelydiffusible across lipid membranes typically have a water-octanolpartition coefficient in a range between −1 and 1. Hyperpolarizedt-butanol and 2-methyl-2-butanol are relatively small and have logwater-octanol partition coefficients close to zero, indicating that theyhave almost equal affinity for water and lipids. The alcohols thereforeare highly membrane permeable while remaining water soluble, allowingfor penetration of the blood-brain barrier and tissue cell membranes.The free permeability also increases the concentration of the agents inthe tissue and lengthens the outflow time constant by up to an order ofmagnitude relative to agents that do not penetrate cell membranes.

In addition, the metabolism and toxicity of t-butanol has beenextensively studied because it is a breakdown product of the fueladditive MBTE. T-butanol is not processed by the alcohol dehydrogenaseenzyme. Instead, the majority of the agent is removed more slowly,likely through the lungs and urine in original form or relatively safemetabolites. Conjugation with glucuronic acid occurs, as does oxidationto 2-methyl-1,2-propanediol, 2-hydroxyisobutyrate, and potentiallyacetone and formaldehyde, but in low concentrations.

There may also be particular advantages to the use of 2-methyl-2-butanolor other hyperpolarized alcohols produced by reaction withpara-hydrogen. The PHIP technique can be performed by a number ofmethods (including field cycling) either before or after incorporationinto the delivery mechanism of choice (e.g., a syringe or powerinjector), thus facilitating relatively fast administration followinghyperpolarization to minimize T₁ decay. In addition, the procedure doesnot necessitate the use of cryogenic apparatus to the same extent thatDNP techniques demand, nor does PHIP require the introduction of a freeradical agent.

T-butanol and 2-methyl-2-butanol are only representative examples of thepresent invention, which is not limited in its application to theembodiments described herein. In particular, numerous other carbon-13enriched alcohols may offer many of the same qualities including, butnot limited to, a high affinity for both water and lipids that maket-butanol and 2-methyl-2-butanol valuable agents for blood flow imaging.Many similar alcohols have configurations that would allow their use inhyperpolarization techniques such as DNP or PHIP. Carbon-13 enrichedethanol, for example, may be an attractive imaging agent because of itssimplicity and the extensive research regarding the toxic effects ofethanol. The agent's relatively short T₁ can be lengthened by replacingthe hydrogen atoms with deuterons. Carbon-13 enriched butanol is anothersimple alcohol whose use as an MRI perfusion agent is encompassed in thepresent invention. While carbon-13 provides a nuclei with a suitablylong T₁, molecules comprised of different nuclei having relatively longT₁ (e.g., T₁ greater than approximately 15 seconds) may be used as well,such as molecules containing nitrogen-15, as the aspects of theinvention are not limited for use with molecules enriched withcarbon-13.

FIG. 5 illustrates a method of using an imaging agent to facilitateimaging blood flow/perfusion, in accordance with some embodiments of thepresent invention. In act 510, the imaging agent is introduced to thesubject being imaged. The term “introduce” refers herein to anymechanism used to admit or otherwise deliver the imaging agent into thesystem of the subject. For example, an imaging agent may be injectedinto the subject, for example, into a vein or artery of the subject.Alternatively, the imaging agent may be introduced via inhalation of theimaging agent by the subject. Other mechanisms for introducing theimaging agent, such as orally or other methods of ingestion, may beused, as the aspects of the invention are not limited in this respect.The imaging agent may be a carbon-13 enriched alcohol (e.g., any of thevariations of molecules illustrated in FIGS. 1A-1D or FIGS. 3A-3B), or amolecule that has a relatively long T₁ and is generally diffusibleacross lipid membranes, as the aspects of the invention are not limitedin this respect.

In acts 520 and 530 the subject may undergo MRI imaging. For example,excitation signals adapted to induce the NMR effect may be provided to aregion of interest on the subject (e.g., a region for which bloodflow/perfusion imaging is desired), and the resulting NMR signalsemitted from the region of interest detected. Imaging of the region ofinterest can be performed using any MRI methods for acquisition of oneor more images at particular time intervals after introducing theimaging agent to the subject and/or using any MRI scanning equipment.Modeling of the time dependence and its relationship to the obtained NMRsignal may be employed to calculate blood flow. Other methods formeasuring blood flow/perfusion from the MR data may used as well, as theaspects of the invention are not limited in this respect.

The present invention may also facilitate the expansion of magneticresonance screening using devices that are smaller, less expensive,and/or more specialized than existing MRI equipment. Hyperpolarizationtechniques can produce large signals even at lower field strengths. Oneembodiment of the present invention is therefore the use of carbon 13enriched alcohols for blood flow imaging in conjunction with specializedscreening devices, including but not limited to mammography apparatus,that rely on weaker magnetic fields than conventional MRI unitstypically generate.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing. In particular, thecompounds described above are merely exemplary and other compounds maybe used, as the aspects of the invention are not limited in thisrespect. Accordingly, the foregoing description and drawings are by wayof example only. Also, the phraseology and terminology used herein isfor the purpose of description and should not be regarded as limiting.The use of “including,” “comprising,” or “having,” “containing”,“involving”, and variations thereof herein, is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

Various aspects of the present invention may be implemented inconnection with any type MR imaging equipment of any configuration. Inparticular, subjects to which any one or combination of imaging agentsdescribed herein may be imaged using any imaging equipment capable ofgenerating excitation signals adapted to invoke an NMR effect andcapable of detecting emitted NMR signals. No limitations are placed onthe implementation and/or configuration of the imaging equipment.Similarly, any type of blood flow/perfusion imaging method may be usedto obtain images of blood flow/perfusion in a subject to which animaging agent has been introduced. In addition, any type of mechanismmay be used to introduce a selected imaging agent, as the aspects of theinvention are not limited for use with any particular method ofintroduction.

Use of ordinal terms such as “first”, “second”, “third”, etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing”, “involving”, andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is: 1-3. (canceled)
 4. A method of imaging blood flow ina region of interest of a subject using magnetic resonance imaging(MRI), the method comprising: introducing an imaging agent into thesubject, the imaging agent including a carbon-13 enriched alcohol;providing at least one excitation signal to the region of interest, theat least one excitation signal configured to invoke a nuclear magneticresonance (NMR) effect at least in the introduced imaging agent; anddetecting an NMR signal emitted by the region of interest in response tothe at least one excitation signal.
 5. The method of claim 4, whereinthe imaging agent includes a hyperpolarized carbon-13 enriched alcohol.6. The method of claim 5, further comprising an act of hyperpolarizingthe carbon-13 enriched alcohol using dynamic nuclear polarization (DNP).7. The method of claim 5, further comprising an act of hyperpolarizingthe carbon-13 enriched alcohol using para-hydrogen induced polarization(PHIP).
 8. The method of claim 5, wherein the carbon-13 enriched alcoholincludes tert-butyl alcohol (t-butanol).
 9. The method of claim 8,wherein a molecule of the t-butanol imaging agent is of the form:


10. The method of claim 8, wherein the t-butanol imaging agent isdeuterated and a molecule of the deuterated butanol imaging agent is ofthe form:

wherein D denotes a deuterium.
 11. The method of claim 8, wherein amolecule of the t-butanol imaging agent is of the form:


12. The method of claim 8, wherein the t-butanol is deuterated and amolecule of the deuterated butanol imaging agent is of the form:

wherein D denotes deuterium.
 13. The method of claim 5, wherein thecarbon-13 enriched alcohol includes 2-methyl-2-butanol.
 14. The methodof claim 13, wherein a molecule of the 2-methyl-2-butanol imaging agentis of the form:


15. The method of claim 13, wherein the 2-methyl-2-butanol imaging agentis deuterated and a molecule of the deuterated 2-methyl-2-butanol is ofthe form:

wherein D denotes deuterium.
 16. The method of claim 13, wherein amolecule of the 2-methyl-2-butanol imaging agent is of the form:


17. The method of claim 13, wherein the 2-methyl-2-butanol image agentis deuterated and a molecule of the deuterated 2-methyl-2-butanolimaging agent is of the form:

wherein D denotes deuterium.
 18. The method of claim 4, wherein theintroducing the imaging agent includes injecting the imaging agent intothe subject.
 19. The method of claim 4, wherein the introducing theimaging agent includes inhalation of the imaging agent by the subject.20. The method of claim 4, wherein the introducing the imaging agentincludes ingestion of the imaging agent by the subject.